Cellular communication systems ring called parties by sending ring-alert messages on a cell's broadcast channels. Ring-alert messages are directed to particular subscriber units. Subscriber units monitor broadcast channels to determine when a ring-alert message is directed to a particular subscriber unit.
In cellular systems, ring-alerts are generally broadcasted in all cells of an entire geographic region where the called-party may reside. Broadcasting each ring-alert within each cell requires a certain amount of additional energy. In terrestrial cellular systems, there is no need to conserve energy because the energy supply is virtually unlimited.
However, in a satellite communication system, where subscriber units receive ring-alerts from satellite communication nodes, it is desirable to conserve energy by broadcasting ring-alerts in as few cells or antenna beams as possible, since a satellite's energy supply is extremely limited. A satellite's power is generally limited to that supplied by its solar panels and its batteries. Energy expended for ring-alerts reduces the amount of energy available for other communications.
Furthermore, in non-geo-synchronous multi-beam satellite communication systems, is extremely difficult to acquire a large number of mobile subscriber units because the location of a satellite's antenna beams is continually changing with respect to the earth's surface. This is true even though the locations of the mobile subscriber units can be provided to the satellite. This problem increases substantially when the satellites are in a low-earth orbit (LEO) because the relative velocity between the satellite and the subscriber unit is significant. For initial acquisition of a mobile subscriber unit, ring-alerts and pages are desirably directed within the proper antenna beam of the proper satellite in a time-efficient manner to provide satisfactory service to users placing calls, and to conserve satellite energy.
Thus what is needed are a method and apparatus that quickly and accurately determines a location of a mobile subscriber unit within a specific antenna beam. Moreover, what is needed are a method and apparatus that quickly maps an antenna beam pattern with a geographic location of a mobile subscriber unit so that acquisition can be achieved quickly and accurately. What is also needed are a method and apparatus to conserve energy onboard a satellite communication station that issues ring-alerts and pages. Furthermore, what is needed are a method and apparatus for reducing the amount of power expended in transmitting ring-alerts in satellite-based communication systems. What is also needed are a method and apparatus for broadcasting ring-alerts and pages in as few antenna beams as possible in a satellite communication system.